Crater type glow discharge tube



Feb. 19, 1935.. R wlLUAMS 1,991,480

CRATBR TYPE GLOW DISCHARGE TUBE Filed Dec. 22, 1932 INVENTOR nwlwimbh; UM

Patented Feb. 19, 1935 PATENT OFFICE CRATER TYPE GLOW DISCHARGE TUBE Richard Melvin Williams, Rutherford, N. signor to Richard Howland Ranger,

J., as- Newark,

Application December 22, 1932, Serial No. 648,845

12 Claims.

The present invention relates to glow discharge tubes of the crater type designed to be used, for example in television and film recording.

In tubes of this type as usually constructed the effective cathode emission surface is of small area and the permissible exciting current is small, usually of the order of 0.05 of an ampere. The voltage may approximate 150-180 volts. When due to fluctuations in the voltage, the current increases beyond a certain limit, sputtering occurs and the cathode wears away, the wear being greatest in the region closest to the anode where the resistance is least. As a result the shape of the crater is changed and the efliciency of the tube is impaired. The sputtering also causes a blackening of the glass and absorption of the contained gas.

The present invention has for an object to provide an improved tube of the crater type. It aims to so form the cathode that upon increase of voltage the cathode discharge will be distributed over a larger area of the cathode while retaining the characteristics of a crater discharge glow tube, thereby permitting a larger total current, while an undesirable increase of current per unit of area is avoided. The invention aims also to provide a cathode such that as the cathode wears due to such sputter as occurs a crater of eifective form will be maintained.

The cathode of the preferred embodiment of the invention is formed to provide an extended discharge area so disposed around the incandescent glow region that upon increase of voltage the discharge may be distributed over a greater area without too great increase of voltage and current at one limited region. The resulting increase of current, because of the resistance in the usual circuit, tends to prevent the building up of a voltage sumcient to cause sputtering.

The nature and objects of the invention will be better understood from a consideration of a particular illustrative embodiment for the purposes of description of which reference should be had to the accompanying drawing forming a part hereof and in which- Figure 1 is a side view of a crater type glow discharge tube constructed in accordance with the invention,

Fig. 2 is a detail view showing the construction of the crater cathode,

Fig. 3 is a sectional view showing a crater cathode of usual type, the worn condition after use being indicated in dotted lines, and

Fig. 4 is a sectional view of a cathode of the shape shown in Fig. 3 but with a protecting shield applied thereto in accordance with the invention.

In the preferred illustrative embodiment shown in Figs. 1 and 2, the tube comprises a crater type cathode 5 and anode 6 having a central aperture 7. The space between the anode and the cathode should be so adjusted that minimum starting voltage is obtained. The cathode is supported in a porcelain cup 8 having a porcelain cover 9 over which the anode 6 fits in the form of a cap holding the cover 9 in place and providing a point of attachment for the mechanical supports. As shown, the porcelain cup with the cathode and anode are supported by a frame consisting of four equally spaced wires, two of which indicated at 10 constitute electrical conductors leading to the anode. The cathode is connected to its conductor through the stem 11 forming a part of the porcelain cup.

The cathode 5 is formed of a plurality of disks 12, 13, 14, 15 having central apertures 16, 17, 18, 19 and a disk 21 having a central hole for the connection of an electrical conductor. All discharge spaces within the cathode, that is to say the spaces between adjacent disks and the spaces of the apertures in the disks should be large enough to permit a free current, in other words they should provide a non-restricted discharge region throughout the effective life of the tube. To this end the distance between plates should be at least twice the width of the normal cathode dark space. Inasmuch as the length of the mean free path is a function of the kind of gas, the gas pressure and the current density, as well as of the normal cathode drop coeiiicient, it will change to some extent as the gas pressure in the tube decreases after a period of use. Accordingly it is not suflicient to make the space the optimum calculated for the conditions existing when the tube is new, but a compromise should be adopted. Probably the optimum distance, at least between the first two disks 12 and 13, is two to four times the length of the mean free path.

In the construction shown no attempt has been made to vary the distance between the different disks with the object of changing the proportionate relation between the increases of voltage and the consequent increase of current nor has an attempt been made to accomplish the same object by making the different disks of metal of dlflerent normal cathode drop coefficient for the reason that this has not been considered necessary for the purposes for which the present tube is more particularly designed.

The structure described is selected as convenient for manufacturing and as providing the desired extended area in the form of what may be termed inwardly directed fins or projections. There are certain advantages in this particular structure, but in the broader aspects of the invention it should be considered as merely an illustrative embodiment of the principles of the invention.

The present invention provides further for protecting the mouth of the crater against wearing away at its outer edge, that is to say at the region where crater type cathodes are subjected to the greatest deterioration due to sputtering. This is accomplished by forming the body of the oathode or more especially the current discharge surfaces, of material of relatively lower normal cathode drop coefiicient and protecting the mouth of the crater with material having a higher normal cathode drop coefiicient, low vapor pressure and high resistance to bombardment effects. In the cathode of Fig. 2, this is accomplished by a shield 23 which effectively protects the cathode at the region which is usually the region of greatest wear. Conveniently this shield may be in the form of a sleeve extending completely through the disk 12 and therefor protecting the whole area of the aperture. The outer and inner ends of the sleeve are formed to engage the edges of the disk to hold the sleeve in place. In the particular structure shown the ends of the sleeve are split to form prongs 24 which are bent over against the outer and inner faces of the disk 12, it being considered unnecessary in commercial practice to extend the protecting material over the edge as a continuous fiange on either face of the disk, although this may be done to give a certain further protection, if desired. All of the disks may be of the same material unless it is desired to use materials of different cathode drop coefiicient to provide a corresponding distribution of the resistance.

Materials of low cathode drop coeflicient and which are suitable for the cathode include for example aluminum and magnesium. Materials of high cathode drop and low vapor pressure which are suitable for the shield include for example tantalum, molybdenum or tungsten. These materials also have high resistance to the effect of bombardment.

In Fig. 3 is shown an iron cathode of usual form. The manner in which the cathode 25 of this type wears. due to sputtering is indicated in dotted lines 26 which show the shape of the face of the cathode after extended use. In Fig. 4 a cathode 2'7 of the same type is illustrated but with a protecting shield 28 applied at the mouth of the crater in accordance with the principles of the present invention. Such a cathode represents a substantial improvement over the cathode shown in Fi 3.

The'dimensions of the cathode shown in Fig. 2 may be calculated in accordance with the principles outlined. In one example the cathode of aluminum in a tube filled with helium gas at a pressure of 30 to 40 mm. has the following dimensions- Inches Diameter of the disks 0.895 Vertical distance between disks 0.100 Thickness of disks 0,105 Diameter of the aperture of disk 12 0.080 Diameter of same as shielded 0.060 Diameter of aperture of disks 13, 14, 15 0.120 Distance between anode and cathode 0.030

Such a tube upon test showed a useful life of 600 hours when operated at approximately 140 volts and milliamperes. After500 hours the striking voltage rose due apparently to decrease of gas pressure. This tube could be reconditioned for use by introduction of gas. Tests upon another tube of this type showed for a voltage rise from to or volts a current increase from 50 to 300 milliamperes.

The foregoing particular description is illustrative merely and is not intended as defining the limits of the invention.

I claim:

1. A crater type glow discharge tube comprising an anode, a cathode of material having a relatively low normal cathode drop formed with a crater, a lining member of metal of the group consisting of tantalum, molybdenum and tungsten fitted within the mouth of the crater.

2. A crater type glow discharge tube comprising an anode, a cathode of metal of the group consisting of aluminum and magnesium formed to provide a crater, and a lining member of metal of the group consisting of tantalum, molybdenum and tungsten fitted within the mouth of the crater.

3. A crater type glow discharge tube comprising an anode, a cathode of metal of the group consisting of aluminum and magnesium formed to provide a crater, the cathode being formed to provide a plurality of inwardly directed fins spaced from each other at least by a distance substantially twice the normal cathode dark space and a lining member of metal of the group consisting of tantalum, molybdenum and tungsten fitted within the mouth of the crater.

4. In a crater type glow discharge tube, a crater cathode of material of relatively low normal cathode drop comprising a plurality of parallel disks having aligned perforations forming the crater, and a lining of material of relatively high normal cathode drop within the perforation of the outermost disk to protect the said disk against sputtering at the mouth of the crater.

5. In a crater type glow discharge tube, a crater cathode of metal of the group consisting of aluminum and magnesium, said cathode being formed with a plurality of projections extending inwardly toward the crater center and spaced sufficiently to provide a non-restricted discharge region, and a lining member of metal of the group consisting of tantalum, molybdenum or tungsten fitting the mouth of the crater to protect the cathode against sputtering.

6. In a crater type glow discharge tube, a crater cathode of material having a relatively low normal cathode drop, said cathode being formed with a plurality of projections extending inwardly toward the crater center and spaced sufficiently to provide a non-restricted discharge region, and a lining member of material having a relatively high normal cathode drop fitting the mouth of the crater to protect the cathode against sputtering.

'7. A crater type glow discharge tube comprising an anode, a cathode of metal of the group consisting of aluminum and magnesium formed to provide a crater, the cathode being formed to provide a plurality of inwardly directed fins spaced from each other sufiiciently to provide a non-restricted discharge region.

8. In a crater type glow discharge tube, a crater cathode of metal of the group consisting of aluminum and magnesium, said cathode being formed with a plurality of projections extending inwardly toward the crater center and spaced suiiiciently to provide a non-restricted discharge region.

9. In a crater type glow discharge tube, a crater cathode comprising a plurality oi parallel disks having alined perforations forming the crater, said disks being spaced to provide a nonrestricted discharge region.

10. In a crater type glow discharge tube an anode, a crater cathode and insulating material therebetween, said cathode comprising at the mouth of the crater a material of relatively high normal cathode drop and around the major efiective portion of the crater a material of relatively low normal cathode drop.

11. A crater type glow discharge tube comprising a crater cathode of material having a relatively low normal cathode drop, said cathode being formed to provide a central discharge region and a plurality of projections extending inwardly toward said central discharge region and spaced from each other suiliciently to provide a non-restricted discharge region, and a cover element constituting a light shield having an aperture registering with the central discharge region.

12. A crater type glow discharge tube comprising a crater cathode of material having a relatively low normal cathode drop, said cathode being formed to provide a central discharge region and a face having a single aperture registering with said central discharge region together with a plurality of projections extending inwardly toward said central discharge region and spaced from each other sufliciently to provide a non-restricted discharge region.

RICHARD MELVIN WILLIAMS. 

